The present invention relates to a spectrum analyser of the light absorption of a solution, particularly for simultaneously dosing a number of compounds dissolved in a solvent, such as for example blood. In this specific medical problem, the measurement of the water exchanged in the organs and particularly in the lungs makes it possible to diagnose the size of edemas in patients and the degree of shock in the victims of accidents.
The presently used method consists of injecting into a vessel upstream of the organ a small amount of heavy water which has been made tonic, i.e. biologically compatible by adding chemical salts and containing a dye such as indocyanine green and then measuring in the blood downstream and as a function of time the concentration leaving the said two substances. In this way, two elution curves are obtained, each having a rapid rise, followed by a slower fall. The capillary walls are impermeable to the dye, but the heavy water passes through them and is exchanged with the water in the tissues, which delays the development of its elution curve compared with that of the indocyanine. The utilization of this difference between the two curves makes it possible to calculate the exchangeable water mass in the organ being studied.
In this special application, it is known to use two independent apparatuses (spectrometers or colorimeters) arranged in series, one for dosing the indocyanine green to 0.8 micron in the very near infrared region and the other for dosing the heavy water in the infrared region at 4 microns.
Spectrometers used for measurements in the visible and infrared regions comprise a light source (tungsten filament lamp or gas lamp for the visible range and incandescent solid source for the infrared), a dispersion system (prism or network) and a detector which is sensitive on the range of wavelengths for which the apparatus was designed (photomultiplier for the visible range and thermal receiver for the infrared). The sample contained in a transparent cell, when a liquid is used, is placed either between the source and the dispersion system (general case in infrared spectrometers) or between the dispersion system and the detector (general case) invisible or ultraviolet spectrometers. Appropriate electromechanical systems make it possible to pass the dispersed light to the detector and record the amplified response of the latter. Measurements carried out with an without a fitted sample make it possible to determine the absorption spectrum of the latter.
In the case where the application is limited to measurements with a single wavelength, simpler instruments (colorimeters) carrying interferential or other filters instead of the dispersion system can be used.
From the quantitative standpoint, the transmission T of the sample at a given wavelength .lambda. is equal to I/I.sub.O, I.sub.O being the incident light intensity on the sample and I the transmitted intensity. Absorption is equal to (I.sub.O -I)/I.sub.O.
The absorbance A or optical density d is the logarithm of the inverse of the transmission: EQU A=d=Log (1/T)=Log (I.sub.O /I)
At present, there are two different types of spectrometers making it possible to measure T and/or d and also they differ as regards their complexity and in the precision of measurement which they can provide.
One of these spectrometers operates with a single beam and the other with two beams.
(a) Single beam spectrometers make it possible to determine the transmission T on the basis of two consecutive measurements. The first measurement, that of I.sub.O is carried out without a sample to be analysed. The second measurement is performed after introducing the sample into the beam. The transmission T is determined for each wavelength of the spectrum and the precision obtained on T is dependent on the conditions under which these two consecutive measurements were performed. Thus, during the measurements there can be variations in the stability of the source, the sensitivity of the receiver, the composition of the atmosphere and the amplification factors of the different components of the apparatus and each of these variations in a source of error. Moreover, the receiver and the amplifier must have linear transformation characteristics, as well as a constant response throughout the measurement. These conditions are difficult to respect and in addition such spectrometers are not very convenient.
(b) In double beam spectrometry, the radiation transmitted by the sources is divided before or after dispersion into two identical beams which pass through the sample and a reference control element of the same time. After passage through the sample and the reference element, the transmitted energies are measured over very short periods, which correspond to the modulation frequency of the beams by means of reflecting or non-reflecting sectors moving in such a way as to intersect the two beams. The transmitted energy is measured on the basis of detectors located on the path of the transmitted beam and said detectors make it possible to measure an energy difference corresponding to the more or less large light absorption of the sample. At the output of the detectors, an electromotive force appears, which varies periodically as a function of a multiple of the displacement frequency of the reflecting and non-reflecting sectors. Thus, an alternating current whose magnitude and phase correspond to the energy difference between the two beams circulates in the detection circuit.
As a function of the absorption value of the sample, it is often necessary to compensate the intensity of the reference beam, said compensation being of two types:
Optical compensation in which a comb or wedge-shaped diaphragm is used, whose displacement controls the response of the recorder indicating the transmission of the sample as a percentage. In this type of compensation, the photometric recording quality is dependent on the linearity of the response of the compensation diaphragm.
Electronic compensation in which the two beams are compared electronically. The intensity of the reference beam is fixed at a constant value by means of a servomotor which controls the opening of the corresponding diaphragm. In this case, the transmitted intensity I is proportional to T and can be directly recorded.
An apparatus of the type described hereinbefore has the disadvantage of not permitting individually the simultaneous determinations of a number of components contained in the same sample and in different wavelength ranges. This is for example the case with simultaneous dosages of uranium hexafluoride and chlorine trifluoride or of water and carbon dioxide. They also do not now permit the simultaneous determination of heavy water at 4 microns and idocyanine green at 0.8 micron in the blood, which itself has a very high absorption in these ranges.
A double beam apparatus especially designed for measurements at the two above wavelengths could be used for dosing the heavy water and then the indocyanine with reference to blood, but not simultaneously. This is a serious disadvantage when it is desired to compare the elution curves of these two products simultaneously injected upstream of the observation point. In this case, it is necessary to design an apparatus which can operate at the two wavelengths chosen, but this leads to a large number of problems which are difficult to overcome and resulting in particular from two factors. The first is the considerable difference in the wavelength necessary for dosing the two products in question (0.8 and 4 microns respectively) and the second is the very high absorption of the blood at these two wavelengths.
In order to solve the problem of dosing two components, bearing in mind the disadvantages referred to hereinbefore, it is standard practice to use dosing apparatuses, such as spectrometers or colorimeters coupled in series. The imprecision of measurement when using such apparatuses is increased by the fact that it is necessary to use two measuring cells, interconnected for example by a flexible tube. Moreover, the dimensions of the cells to be used are linked with the optical and electronic characteristics of the apparatuses chosen, so that great differences in these dimensions can deform the elution curves with respect to one another. Thus, the use of two series-arranged apparatuses with two cells of different dimensions, connected by a flexible tube make it difficult to establish a common time base with the considerable precision required for kinetic studies.
However, the sensitivity and precision of measurement of such apparatuses, taken individually, may be able to satisfy the requirements of the problem, i.e. the measurement of small variations on an intense absorption background as a result of the choice of sources and detectors having better performances at the selected wavelengths.
In the case of detectors, the problem is solved relatively well by choosing, for example, selenium pyroelectric elements. As this type of detector has in practice the same specific detectivity between 0.2 and 35 microns, it can be used both for measurements in the visible region and in the infrared region.
The problem is more complicated in the case of light sources. Thus, the intensity and transmission range vary with the type of source selected. It is possible to select incandescent sources using aluminium oxide-based ceramic materials, whose operating temperatures are between 1500.degree. and 1800.degree. K. and whose emissivity values differ relatively greatly from those of the black body for very near and far infrared. It is also possible to use tungsten filament lamps for the visible range and hydrogen and deuterium spectral lamps for the ultraviolet.
These briefly reported choices make it possible to improve the sensitivity and precision of the measuring apparatus, but the problem still exists when it is a question of following the time evolution of the concentration of each of the components and of comparing their elution curve. Thus, the solution consisting of connecting apparatuses in series has very serious practical difficulties particularly as a result of using separate cells.
Frequently in known double beam spectrometers, the light energy concentration on the solution to be analysed is low due to the arrangement of the input mirrors making it possible to divide the beam transmitted by the source and of the arrangement of modulation means for the divided beams with respect to the cell or the container containing the solution to be analysed. Usually, these arrangements are such that it is necessary to place between the different organs means for deflecting the optical paths of the two beams, such as for example mirrors.